1. Technical Field
The present disclosure relates to a mechanical or non-hydraulic variable camshaft timing device for an internal combustion engine.
2. Background Art
Variable camshaft timing (VCT) devices may be used to change the phase relationship between the crankshaft and the camshaft lobes that control actuation of the gas exchange valves. In general, control of the phase relationship facilitates better management of fuel economy, performance, and emissions by providing less valve overlap at idle and low engine speeds for good combustion stability, and more valve overlap at higher engine speeds for better power and performance. Pressurized engine lubricating oil is often used to hydraulically actuate VCT devices. However, temperature-related viscosity changes often present challenges for consistent performance of hydraulically actuated VCT systems. In addition, hydraulically actuated VCT systems may require higher oil pressure and a correspondingly larger, heavier oil pump with higher parasitic losses than otherwise required to provide adequate engine lubrication, resulting in lower overall engine efficiency and fuel economy.
A mechanically actuated phaser for adjusting the timing between a camshaft and crankshaft as disclosed in US 2003/0159670 uses solenoid-actuated rotation of a worm gear to vary the angular relationship between the camshaft drive sprocket and a camshaft drive gear. This strategy recognizes that a worm gear with a high mechanical advantage will rotate when torque is applied to the worm, but a constant torque applied to the worm gear will not produce rotation of the worm so that the device can maintain a selected position. However, US 2003/0159670 does not recognize that the dynamic loading associated with valve actuation creates a fluctuating load applied to the worm gear that can cause rotation of the worm similar to the manner in which an under-torqued bolt can vibrate loose. As such, the lash necessary for operation of the worm in this arrangement makes it difficult to hold a constant position when subjected to the dynamic loading of the camshaft during operation of the intake and/or exhaust valves and may require constant adjustment by the actuating solenoid. In addition, packaging constraints with an internal worm gear meshing with internal ring gear teeth require small components that may encounter reliability/durability issues.